$ sudo update-desktop-database
Warning in file "/usr/share/applications/firefox.desktop": usage of MIME type "x-scheme-handler/http" is discouraged (the use of "x-scheme-handler" as media type is strongly discouraged in favor of a subtype of the "application" media type)
Warning in file "/usr/share/applications/firefox.desktop": usage of MIME type "x-scheme-handler/https" is discouraged (the use of "x-scheme-handler" as media type is strongly discouraged in favor of a subtype of the "application" media type)
Warning in file "/usr/share/applications/firefox.desktop": usage of MIME type "x-scheme-handler/ftp" is discouraged (the use of "x-scheme-handler" as media type is strongly discouraged in favor of a subtype of the "application" media type)
Warning in file "/usr/share/applications/firefox.desktop": usage of MIME type "x-scheme-handler/chrome" is discouraged (the use of "x-scheme-handler" as media type is strongly discouraged in favor of a subtype of the "application" media type)

If I remove the x-scheme-handler types from /usr/share/applications/firefox.desktop and rerun ‘update-desktop-database’, the problem goes away, and gnome-open lets me use my preferred web browser again.

I now have chromium-browserchromium-browser 9.0.587.0~r66374-0ubuntu1~ucd~dev2 from ppa:chromium-daily-dev, which has already been updated to include
MimeType=text/html;text/xml;application/xhtml_xml;x-scheme-handler/http;x-scheme-handler/https;
in /usr/share/applications/chromium-browser.desktop. However, this isn’t sufficient to cause Chromium to act as the default browser, either when I select it in gnome-default-applications, or when I tell Chromium to make itself the default browser in its Preferences page.

Warning in file "/usr/share/applications/chromium-browser.desktop": usage of MIME type "x-scheme-handler/http" is discouraged (the use of "x-scheme-handler" as media type is strongly discouraged in favor of a subtype of the "application" media type)
Warning in file "/usr/share/applications/chromium-browser.desktop": usage of MIME type "x-scheme-handler/https" is discouraged (the use of "x-scheme-handler" as media type is strongly discouraged in favor of a subtype of the "application" media type)

* 140_backport_default_internet_apps.patch:
- Backport default Internet app panel from 3.0 to 2.32. This lets one
set the new-style glib url handlers for web and mail.
* 141_set_default_internet_apps.patch:
- On startup, set the web and mail combo boxes to the current glib
value. Patch sent upstream.
* 142_fix_icons_for_default_apps.patch:
- Fix icon handling for web and mail combo boxes. Patch sent upstream.
* 143_write_default_internet_apps_to_gconf.patch:
- Until we update GNOME to 3.0, write back the current glib url
handlers to gconf for apps still directly using such keys rather than
the glib functions for launching apps.

I think a solution for Chromium will be to use "new" mimes only on Natty, and revert to the old ones on Lucid/Maverick.
(It does not affect the current 8.0.552 branch, but it will affect 9.0.593+ branch when it'll be released. Also, it affects beta/dev/nightly ppas).

In addition to this though, I had to also fix xdg-mime to use the users correct default mimetype handler settings (it was using ~/.local/share/applications/defaults.list, which is wrong. It'e meant to be ~/.local/share/applications/mimeapps.list, according to http://www.freedesktop.org/wiki/Specifications/mime-actions-spec and this is what everything else already uses)

Chromium is shipping its own xdg-mime and xdg-settings, so I've assigned myself to that too